1. Field of the Invention
The present invention relates to a dielectric waveguide having a dielectric strip interposed between a pair of parallel flat electrically conductive plates, for propagating millimetric waves therethrough.
2. Description of the Prior Art
Electromagnetic waves which are polarized parallel to the wall surfaces of parallel metallic plates are blocked and cannot propagate along the parallel metallic plates if the distance between the parallel metallic plates is half the wavelength of the electromagnetic waves or less. When a dielectric strip is inserted between the parallel metallic plates, however, electromagnetic waves can propagate along the parallel metallic plates, but radiative waves are completely suppressed by the cut-off effect of the parallel metallic plates. Based on such principles, there has been proposed, as shown in FIGS. 1 and 2 of the accompanying drawings, a nonradiative dielectric waveguide (NRD) having a dielectric strip 3 sandwiched between parallel metallic plates 1, 2 (see Journal of Electronic Information Communications Society, C-1, Vol. J73-C-1, No. 3, pages 87-94, published March 1990).
Other conventional nonradiative dielectric waveguides have a termination as shown in FIGS. 3 and 4 of the accompanying drawings.
The nonradiative dielectric waveguide shown in FIG. 3 comprises a pair of parallel flat plates 1, 2 and a dielectric strip 3 sandwiched between the parallel flat plates 1, 2. Resistive films 4 of NiCr with tapered ends 41 for attenuating the reflection of input electromagnetic waves are applied to respective opposite sides of the dielectric strip 3. The tapered ends 41 serve as a termination for eliminating reflections. However, since attenuation factor of electromagnetic waves per unit length along the dielectric strip 3 is relatively small, the termination is relatively long.
The nonradiative dielectric waveguide shown in FIG. 4 also comprises a pair of parallel flat plates 1, 2 and a dielectric strip 3 sandwiched between the parallel flat plates 1, 2. The dielectric strip 3 is divided into two layers parallel to the parallel flat plates 1, 2, and a resistive film 5 with a tapered end 51 being inserted between the layers of the dielectric strip 3. The tapered end 51 serves as a termination for eliminating reflections. The attenuation factor of electromagnetic waves per unit length along the dielectric strip 3 is greater than, and hence the termination is shorter than the case with the nonradiative dielectric waveguide shown in FIG. 3. However, the nonradiative dielectric waveguide shown in FIG. 4 fails to have uniform characteristics because of a complex process required to manufacture the nonradiative dielectric waveguide, i.e., separating the dielectric strip 3 into two layers, placing the resistive film 5 between the layers, and bonding them together.
Generally, nonradiative dielectric waveguides have such an electromagnetic field intensity distribution that the electromagnetic field is greatest in the dielectric strip and becomes smaller in a direction away from the dielectric strip depending exponentially on the distance from the dielectric strip. Since the nonradiative dielectric waveguides shown in FIGS. 3 and 4 have the respective resistive films 4, 5 directly combined with the dielectric strips 3 where the electromagnetic field intensity is high, the resistive films 4, 5 are highly exposed to electromagnetic waves, and electromagnetic wave reflections tend to vary greatly with small changes in the shape of the resistive films 4, 5. Accordingly, it has been difficult to obtain desired attenuation and reflection characteristics for the nonradiative dielectric waveguides shown in FIGS. 3 and 4, particularly uniform attenuation and reflection characteristics when the nonradiative dielectric waveguides shown in FIGS. 3 and 4 are mass-produced.